1. Field of the Invention
This invention relates to ignition systems for creeping spark plugs and more particularly to an ignition system for a semi-creeping spark plug or a full-creeping spark plug for producing a spark discharge at least one portion of which slidably moves along a creeping discharge path.
2. Description of the Prior Art
In the above mentioned kind of spark plugs, carbon is often deposited on the creeping discharge path depending on operating conditions of an engine which makes use of the spark plug.
In order to remove such deposited carbon, heretofore it has been the common practice to utilize a spark energy of the spark discharge produced along the creeping discharge path when a mixed gas is ignited so as to scatter or burn the deposited carbon. Such conventional carbon removing system is generally called as an electrical self-purifying system.
Experimental tests on such electrical self-purifying system have yielded the result that the carbon is mainly removed by a capacitive discharge energy produced at the beginning of the spark discharge, and that an inductive discharge energy immediately followed by the capacitive discharge energy becomes jumped up from the creeping discharge path, thereby exhibiting no effect of removing the carbon deposited on the creeping discharge path.
The simplest and most effective way of increasing the capacitive discharge energy is to repeat the spark discharge for a number of times.
Further experimental tests have demonstrated the result that the amount of carbon to be deposited on the creeping discharge path becomes significantly changed in dependence with the operating condition of an engine, and that even though the electrical self-purifying system is improved, such improvement is not sufficient to provide an ignition system which can effectively prevent the creeping discharge path of the spark plug from being deposited with carbon irrespective of all of the operating conditions of the engine.